1. Field of the Invention
The present invention relates to an aromatic polyester obtained from parahydroxybenzoic acid, hydroquinone, terephthalic acid and 2,6-naphthalenedicarboxylic acid.
2. Prior Art
Aromatic polyesters obtained by polymerization of (1) parahydroxybenzoic acid, (2) hydroquinone,(3) terephthalic acid and (4) 2,6-naphthalenedicarboxylic acid have been already known. 
Specifically, U.S. Pat. No. 5,969,083 reports that when number of moles of monomer unit derived from (1) is 53.8 moles per 100 moles of the monomer units derived from (1) to (4) in total which constitute the aromatic polyester; and when the value which is represented by the following formula (I) is 0.2 or 0.4:[(3)]/{[(3)]+[(4)]}  (I)wherein [(3)] and [(4)] represent number of moles of the monomer unit derived from (3) and the monomer unit derived from (4),the resulting aromatic polyester has a low melting point and low viscosity, and thus a molded article obtained therefrom exhibits a high heat distortion temperature (i.e., a deflection temperature under load) and excellent tensile elongation.
Further, Japanese laid open patent publication WO 97/45469 reports that when number of moles of monomer unit derived from (1) is 55.6 moles per 100 moles of the monomer units derived from (1) to (4) in total; and when the value represented by the formula (I) is 0.4, the resulting aromatic polyester has a low melting point, and thus a molded article obtained therefrom is excellent in tensile strength, tensile elongation, flexural strength and a modulus of flexural elasticity, and has a high heat distortion temperature (i.e., a deflection temperature under load).
The present inventors studied on aromatic polyesters containing monomer unit derived from (1) at 54-62 moles per 100 moles of the monomer units drived from (1) to (4) in total which constitute the aromatic polyester, and having the value represented by the formula (I) of 0.23-0.35, and revealed that the molded article obtained from the aromatic polyester has insufficient impact strength.
An object of the present invention is to provide an aromatic polyester which gives a molded article having excellent mechanical strength such as tensile, flexure and the like with excellent heat resistance such as a heat distortion temperature, and having excellent impact strength even with an aromatic polyester obtained from (1) parahydroxybenzoic acid, (2) hydroquinone,(3) terephthalic acid and (4) 2,6-naphthalenedicarboxylic acid, containing the monomer unit derived from (1) at 54-62 moles per 100 moles of the monomer units derived from (1) to (4) in total, and having the value represented by the aforementioned formula (I) of 0.23-0.35